How to open a large .txt file on Android smartphone?

There are a couple of good apps that let you open the large TXT files.
I would advise you to tap on the file you would wish to open and choose ES Note editor as the default app for the file. If you do not have ES File Explorer, you will need to download and install the same on your Android device.
Another app that can perform this task is Quick Edit. The app can be downloaded from the link https://play.google.com/store/apps/details?id=com.rhmsoft.edit. The app has been tested to open files as huge as 2 MB. I hope it should help you open your file.
On the other hand, if you have a full fledged office file viewer - that would be the best choice for opening a text file of that size. In fact, such a full-fledged app will be better than the note editors as it would help you open many office files. I have WPS Office on my phone and I use it for most of my needs.

There are some of the text editors that are designed to open large files. Using those files you can open formatted txt, tsv and other data based text file.

1. Jota Text Editor : If you can't open the text file from the default file explorer notepad. Then this is the text editor that you can open with Jota Text Editor. This text editor reads large text files. All you have to do is tap into the text file and open the options and then open it in Jota.

2. NTW Text Editor Pro: Another text editor that you can use for opening the large file size. It can be used for both editing and reading the large files. It can also offer options for file search, replace text and regex. So it is full featured editor.

3. TextWarrior : It has some of the buffer limitation on how much file size it can read. However it is known to handle large set of files. It allows both editing and reading of the text and other editing file formats.

Apart from these you have apps like -

1. ES Note editor
2. QuickEdit
3. TruTxt reader writer

WP Office and Google Docs can be used to read the text file. However they don't handle the large file over 500MB due to some limitations. You can confirm this as some of the files won't have the scrollbar to read once you try to open files in them.

Note : The capacity to read the large files also depends on the RAM size. If your RAM is low on buffer while the program is open, then it affects the text file read write rate. So make sure that you have more than 1GB RAM if you choose to read over the file size of amount of 100MB and above.

There are a large number of apps available in the Google Play for this purpose. But I don't think 800MB file could be opened in any software in android device. Maximum megabyte docs been opened in few best software's.
1. WPS Office
2. Drive PDF viewer
3. Quick Edit
These are some of the best software apps which could help you to sort out your problem. These software's handle megabyte docs but even then they have limitations in size.